(Taekwondo practitioner for nearly 30 years, running a Taekwondo club)
I would go with the others general advice on getting them started in a club under a qualified instructor. However, I understand there are places where either people are too far from a club or it's financially not feasible, so teaching them the very basics gives them something to work on in the meantime.
So, we normally start like this:
1) Teach them how to clench a fist properly. Then go in to Charyeot (attention stance, fist at the sides, don't flap them/slap them against the legs, feet together, all toes forward), Kyungye (bow, waist to 30 degrees head to 45 degrees, look down NOT at your opponent) and Joonbi (ready, feet one foot length apart, all toes forward, hands come slowly up to solar plexus open and down slowly clenching until they reach the belt not where they stop one fist apart and one fist away from the belt not).
2) We do sitting stance punching - keep the feet two foot lengths apart, all toes facing forward, knees bent but naturally not forced outward. When punching initially focus on getting the hands right, ensuring the twist is in the last 20% of the movement, then add in waist/body twisting.
3) We teach them three basic blocks - low block, rising block and inward block. We do this in steps - prepare, step, block - for each, then move them on to doing all three steps together (so as you're starting to step prepare the hands to the right place, then as you finish and your foot stops, your block stops). All are done in short/walking stance (we don't teach long stace - Apkubi - until after the first belt test).
4) We teach them four basic kicks. The principle is that each kick is attacking an opponent in front from a different attack direction. So a front kick - Ap Chagi (ensuring proper knee bending before AND after kicking) comes in a slightly upward direction, a push kick - Mirroh Chagi (knee to the chest before pushing away with the sole of the foot) comes from a straight forward direction, axe kick - Naeryo Chagi (swings straight up, knee straight at this grade, then point the foot and pull down) comes downward, then turning/roundhouse kick - Dollyo Chagi (knee forward as per front kick, foot and toes pulled back, then turn the standing foot and rotate the leg so the shin is parallel to the floor, kick out then retract the same way) comes from the side.
After that we often go in to footwork, but this is a bit more difficult to describe.
I would advise not making him wear a dobok and certainly don't think about if lessons go on, giving him any belts. If you can purchase a Taekwondo paddle, you can use it to have him kick something.